Ann Patchett fans might remember that she didn't know the first thing about opera when she began Bel Canto, her breakthrough novel of 2002 with a star-crossed soprano at its centre. So she left her Nashville home for the historic teatri of Italy and a period of immersive research (the bel canto technique is derived from a 19th century Italian vocal style). Her rigour paid off - Bel Canto became a feted bestseller, winning the Orange Prize and the PEN/Faulkner award. A young composer was even inspired to write an opera of the same name and subject.

It follows that when Patchett decided to write her latest novel about a woman who journeys into deepest, darkest Amazonia, the author found herself onto a boat juddering along the alligator-infested waters of the river.

The ten-day trek was taken by Patchett two years ago, and much of the material she gathered has been channelled into State of Wonder (Bloomsbury, £12.99). Her sixth novel crosses pharmacology and undiscovered tribes with a twisting adventure narrative. The story takes lab researcher Marina Singh from her hometown in Minnesota along the Rio Negro and into the dangerous, dark heart of the Brazilian rainforest. There she comes face-to-face with her formidable former teacher and brilliant scientist, Dr Annick Swenson, while searching for a perished colleague who took the same route into the jungle.

Patchett's husband, Karl, a fair-weather aide on her research tours (he jumped at the chance of accompanying her to the opera houses) was up for a little adventure in the Amazon. They took a boat to Peru because the only boats available to Brazil were either cruise ships or cockroach-infested tubs. All went well at first. By day four, the heat, the bugs, the stultifying fecundity of the natural world and innumerable lurking terrors of the jungle began affecting morale: "The joke was that for the first three days, my husband kept saying 'I can't believe you brought me here. I'm the luckiest person'. By day seven, he was saying 'I can't believe you brought me here. There are not many husbands that would do this'. You are on the river and all you can see behind, around and above you are trees. It's so claustrophobic, I felt so stifled.

"It was the best of times and the worst of times. There were some really astonishing moments. I was on an open boat with eight others when one man put his hand in the river and picked up a 15ft anaconda and brought it onto the boat. The snake was trying to bite him in the face. It turned out that he was a snake handler so he was giving us a calm lecture on snakes, at which point I really wanted to jump off the boat."

The anaconda fight provided Patchett with one of the most riveting and visceral scenes in the book. The claustrophobia and deadly nature is vividly rendered - the "murky soup" of a river, the "spectacular moths with wings like hankerchiefs", the "microscopic candiru fish that were capable of swimming up the urethra with catastrophic results".

Dr Swenson discovers that the forest's flora and fauna has the secrets of everlasting fertility locked within it. It is this natural secret that the pharmaceutical company at which Marina works hopes to harness, package and sell, as a profitable fertility panacea for the affluent West.

So the apparently timeless journey into the Amazon leaves Marina – aged 42, childless, with an ambivalent older lover – confronting a very topical malaise. The fertility issue, says Patchett, was not one she wanted to rail against. It is a plot device thoroughly blended into the fabric of the mystery and intertwining with aparallel "malaria cure" sub-plot. Patchett calls it a "bait and switch" concept which could have been swapped for something else posing an equivalent moral dilemma. Even so, she found she had strong feelings once she arrived at fertility's door.

"I didn't say to myself 'I want to write a book about fertility'. I thought about the idea of a drug that would make you very thin, or give you everlasting youth or gorgeous hair, and then I came onto fertility.

"Once I got there, I had a lot to say. I'm 47 and I don't have children. I've spent my whole life having people tell me that I will want to have them, maybe not now, but there will be a time when I'll want them. It's a constant, boring conversation about something that is personal. It's certainly one that I have listened to. I think all women have. Either those who don't have children or those who wonder whether they have too many... The best part about getting older is that people don't ask me anymore."

As far as defiantly childless women go, Patchett has a wonderful creation in Dr Swenson, a seventy-something adventurer-scientist who hurls clever rejoiners at the world's body-clock alarmists: "I've never believed the women of the world are entitled to leave every one of their options open for a lifetime," she says at one point, and at another: "Perhaps instead of trying to reproduce themselves, these postmenopausal women who want to be mothers could adopt up some of the excess that will surely be available."

Dr Swenson is, like so many of Patchett's female protagonists, a deeply moral rebel, part Indiana Jones and part Marie Curie, delivering crisp, Aristotelian gems to her protégé, Marina ("Never be so focused on what you're looking for that you overlook the thing you actually find"). It may be tempting to equate the subject matter with Joseph Conrad's Heart of Darkness, and Dr Swenson with its central character, Kurtz. They are both Western renegades surviving in the jungle and exerting control over its indigenous people. But unlike Kurtz, she never loses her moral compass and resists the abuse of power. She promotes non-interventionism and respects the tribe's non-allopathic approach to illness.

Typically, one novel has inspired Patchett's next. "Just at the end of writing this book, I had an idea that's different yet completely the same. I can usually point to one theme [in a novel that inspires the next] and in this case, it is Easter [an indigenous child character] in peril. I thought 'what would it be like to have a whole bunch of those?' It will be in Mexico. There will be a band of drug dealers and a group of children who are kidnapped by pirates. They are in terrible peril but they don't know it."

Patchett's stories are rarely, if ever, driven by a romance narrative. In this instance, she wanted to write an adventure story with a strong woman at its centre, and it was enormous fun to do so - the quickest (14 months in total) and happiest experience of all her books: "Where is the contemporary novel in which a woman is a hero? I really want to read that book. I think we need books in which women are heroes."

The blueprint seems to have been set with Patron Saint of Liars (1992), her debut published at the age of 29, which featured the first of many alternative heroines. Rose Clinton, on realising she has married a man she does not love, simply gets in a car and leaves, Harry "Rabbit" Angstrom style, except that she finds contentment in a community of nuns. Families are refreshingly anti-nuclear across Patchett's work. Orphans abound but few are victims. In Run (2007) a mother dies and the job of bringing up the boys falls to the father; in Taft (1994) a father is estranged from his son; and in this latest book, Marina loses her father at an early age.

Patchett offers an easy answer for this recurring theme, and then a more opaque one: "[As a child] I was fixated with Shirley Temple who was basically an orphan." She goes on to say she is drawn to strong characters, too strong in some senses, to have two living parents. "It's both a nice and an embarrassing thing to have a body of work, because you look back and think 'Jeez, this is obviously my psychology.' Everyone has recurring themes. The trick is not to fight it. I believe you have one deck of cards and you can get an enormous amount of combinations from 52 cards."

The range in her novels attests to that, although there is strong moral grounding to them all. Catholicism has informed her life, she says, but it is the creative aspects of religion that she credits for influencing her novel writing: "Growing up with Catholicism was tremendously helpful. It's a brilliant religion for novelists. It's all about fantastical things happening. Believing in God is really good training in becoming a novelist. You believe in this entity that you have never seen. It's fabulous training for making-up people. Novelists are being God in their way. I make the sun come up and go down again... I design the set, the costumes, the big song and dance number."

Earlier this year, she wrote a moving Granta essay on the lives of the ageing nuns who taught her for 12 years at Catholic school. Patchett has, in a curious way, an air of the Mission herself: she is small, neat, sharp yet guileless, with a vaguely ardent air about her. Admirers make much of the moral strength, warmth and humanity in her stories.

Born in Los Angeles, Patchett moved to Nashville at the age of six. Her fiction has been praised for its distinctively Southern voice, following in the tradition of Eudora Welty and Carson McCullers. Though she left Tennessee at 17, she returned at 30 and stayed, in spite of herself. "I went home to visit my mother and thought 'I never want to live here.' I went on a date with her boss who asked me to marry him on that first date. He was recently divorced, unmoored. We kept dating and I kept saying to myself that we'd break up and then I'd leave. We dated for 11 years and I married him!"

Married life has surprised her, after years of resisting it. "I never lived with him until six years ago. I bought a house three blocks away from him. We were happy. Then he had a heart attack and I thought he might die. Two months later we married. I was really signing up to being his widow.

"But I'm so much happier being married. I realised my passionate desire not to get married had really been an enormous desire not to have a wedding. We didn't tell anyone when we got married. People would ask 'when are you getting married?' and I'd say 'We're married. Didn't I tell you?'"

Their shared home now serves as a haven for her and a refuge for her friends. In a recent article, she wrote tenderly of its rich, regenerative qualities. And after decades of travel, often hectic and extensive, she dreams of never going away, of simply staying put. "As beautiful as the world is, and as much as I understand it's good for me to see other people and stretch my horizons, I'm over it. I could really stay at home forever and make things up."

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